The present invention is directed to a method of consolidating particulate metals, ceramic materials, or combinations of such materials into shaped products of very low porosity. More specifically, this invention provides a means to apply high compaction pressures at temperatures in the range of sintering temperature of the materials being consolidated to achieve essentially complete densification at extremely rapid processing rates as compared with prior art technologies. Moreover, because this invention employs a method of electrothermal heating generally referred to as "charge-resistor" heating, the temperatures that can be obtained by the process of this invention are well above the requirements for even the most refractory and heretofore difficult to densify materials, such as silicon carbide, boron carbide and other very high melting point materials.
Processes for making dense metallic and/or ceramic articles from their powders are well established in industrial practice of the prior art. It is possible, for example, by the method known as "hot pressing," to place the powder between two cylindrical pistons contained within a cylindrical chamber having only a slightly greater diameter than the pistons, and by application of heat and pressure directly to the powder, the powder can be formed into a highly dense cylindrical billet. Because the method applies an external pressure directly to the material at or near its sintering temperature, hot pressing will yield consolidated product of higher density than if the powder material is densified by pressureless sintering. Hot pressing is a relatively simple prior art method to achieve essentially complete densification and is widely used to make many articles in present commercial use. However, because high temperatures are needed to consolidate high melting point metal alloy powders and the recently developed high performance ceramic materials, the dies and plungers used for hot pressing must be made of a material capable of withstanding the required temperature. Graphite is the most practical material and is commonly used in commercial hot pressing of ceramics. To avoid oxidation of graphite tooling, the hot processing apparatus must be enclosed within a controlled atmosphere chamber, and shielded therein by inert gases. Often, a vacuum is also pulled on the chamber containing the hot pressing apparatus to allow for the removal of reactive gases that may evolve during heating of the material.
A major disadvantage of this method of consolidation is that the cycle time or hot pressing increases with temperature, making the process slow at very high temperature. It is a further disadvantage that in order to utilize effectively the space available in a hot pressing furnace, it is necessary to assemble a series of parts into a "stack". This procedure can be intricate and not readily adapted to automated production.
Even more disadvantageous from the standpoint of commercial production of high performance structural ceramics is that it is impossible to use simple uniaxial hot pressing to make parts of complex shape. For example, turbine rotors that have intricate curved shapes and substantial variation in the thickness cannot be hot pressed.
To make an article of complex shape the processing that is generally used involves "preforming" the powder into a partially densified article having the desired shape of the finished part, then further consolidating the pre-form in a second step. Methods commonly used to make the pre-form include die pressing, slip casting, injection molding and extrusion. Pre-forms can then be further consolidated by one or a combination of operations that include cold isostatic pressing, hot pressing, hot isostatic pressing (HIPING), cold and hot forging, hot extrusion and pressureless sintering.
Hot isostatic pressing or HIPING involves sealing a pre-form in an evacuated flexible container and inserting the cladded part into a pressure vessel, then applying pressure while heating to the sintering temperature. The container can be a thin metal sheath that can be welded, evacuated and sealed. If the metal cladding material is properly selected, it will deform to the shape of the preform and transmit pressure uniformly and isostatically to the part being consolidated. Pressure is applied by compression of the gas in the surrounding chamber. For ceramic components that require very high temperatures to achieve consolidation, for example silicon carbide parts, a metal sheath may be impractical and means to encapsulate the part in a glass composition has been developed.
Hot isostatic pressing does provide a means to consolidate metal and ceramic preforms into dense articles of complex shape and there is a growing use of HIPING in commercial production of powdered metal parts and ceramic components. However, the major use of HIP has been to densify cast metals and to consolidate powder metal parts. HIP technology can be applied to ceramic materials including the high temperature non-oxide ceramics, such as silicon carbide; but the costs are very high. Temperatures in excess of 2000.degree. C. are needed at pressures of 30,000 psi or higher. As in hot pressing, HIPING entails long cycle times. Because the cost of high pressure equipment and systems are higher than for hot pressing, HIPING is more expensive. There has therefore been increasing interest in other methods that would allow production of ceramic components of complex shape at lower cost.
Yet further, one method receiving extensive study by ceramicists is pressureless sintering. Pressureless sintering involves making preforms of the part from ultra-fine, sinter-active ceramic powder, for example by injection molding or cold isostatic pressing, then heating the preform to a temperature approaching the melting point of the material(s) to be sintered, causing a shrinkage and consolidation to the desired final size and shape. Pressureless sintering not only can consolidate complex and intricately shaped parts but it offers the obvious advantage of adaptation to continuous and easily automated production with inherently lower cost of mass production.
Despite already noted advantages of the pressureless sintering method, technical factors may limit the ability to utilize this process to make certain high performance structural ceramic components from certain powdered ceramic materials. This is particularly true in the case of the high melting temperature non-oxide ceramic materials such as the nitrides, carbides, and borides. For example, to achieve a near theoretical density part by pressureless sintering of silicon carbide powders, it is mandatory to use a silicon carbide powder that is all substantially below 1-2 micron particle size, and meeting close chemical specifications with regard to contained oxygen, carbon and trace elements.
Deviation from these specifications will lead to failure to obtain an essentially pore-free and homogeneous flaw-free structure absolutely necessary to obtain the high strength characteristics required in aircraft or automotive engine turbine rotors. Thus, the production of suitably qualified powders meeting the rigorous demands for pressureless sintering involves intricate and careful control that results in relatively high cost starting raw material powders. Furthermore, even with the most careful control during production of powders, pressureless sintering at high temperature requires a finite time during which the preformed powder compact must remain at the sintering temperature to allow for full shrinkage and densification. Inherent with the sintering process is the tendency for growth of the grains causing departure from the idealized ultra-fine, equi-axed microstructure that offers the highest material strength. Powders that undergo crystal phase transformation at or near the sintering temperature are particularly susceptible to grain growth and a resulting decrease in strength properties. This effect made more pronounced with increased residence time at the sintering temperature.
It can be noted that very high temperatures are needed to sinter materials that are candidates for high performance engine components. As stated, silicon carbide requires temperatures above 2000.degree. C. to achieve a high degree of densification by pressureless sintering. Although it would be highly desirable from the standpoint of minimizing grain growth to rapidly heat a preform to the sintering temperature and limit the time at temperature to no longer than several minutes, and, possibly to residence times of no more than several seconds, there are a number of practical limitations to doing this by a pressureless sintering process. First, sintering furnaces of conventional design are simply not suited to a quick entry or withdrawal of the material being sintered. More important, however, is the fact that gases are likely to evolve from the preform during heat-up due to a combination of desorption, decomposition and chemical reaction. Too rapid a heat-up rate and associated gas evacuation rate can cause mechanical stresses that would fracture the part. And thirdly, too rapid a heating and/or cooling could also cause fracture due to differential expansion or contraction, referred to as "thermal shock". The limits of temperature rise rate and cooling rate during pressureless sintering are different for different materials and are dictated by the specific material being processed and the size and shape of the part.
However, it should be noted in regard to the limits of temperature rise rate and the speed of the sintering cycle that substantially higher cycle rates are possible when the material is being subjected to pressure, for example as in hot pressing. Pressure minimizes cracking due to gas evolution and, if maintained during the densification cycle it allows a substantially closer approach to achieving theoretical density then by pressureless sintering.
Thus, the present state of the art can be summarized, as follows: the pressureless sintering method for the manufacture of advanced materials by consolidation from powders, and particularly for making high performance ceramics, represents the most attractive method from the standpoint of high volume-low cost production. However, the mechanical properties obtained in pressureless sintered materials are generally not as good as in materials made by application of pressure during sintering, as is the case for hot pressed and/or HIPED products. But these latter methods are either unsuited for making parts of complex shape and/or are very slow and unable to be easily or fully automated; thus, they are very expensive.
It is therefore one material object of the present invention to overcome many of the difficulties of the prior art methods, and more particularly to provide a means for extremely rapid sintering under pressure to achieve consolidated parts of low porosity and having a flaw-free and ultra-fine grain structure.
It is a further object of the present invention to be able to accomplish these basic objectives even with materials that require temperatures well above 2100.degree. C.
It is still further the purpose of the present invention to accomplish the above objectives within seconds using temperature rise rates in excess of 50.degree. C./sec.
Moreover, it is the purpose of this invention to consolidate preformed materials of complex shape under compaction pressures approaching isostatic compaction conditions. It is still further the purpose of this invention to accomplish these objectives using relatively inexpensive apparatus that is simple in construction and operation and that can be readily adapted to automated and continuous high volume production.
It is recognized that the above objectives of the present invention have been addressed by previous workers. In that regard, descriptions of processes intended as improvements in manufacture of parts by consolidation of powders have been described in the technical literature and in previously issued patents. Most pertinent are the patents issued to Lichti and Hofstatter (U.S. Pat. Nos. 4,539,175 and 4,640,711). These workers have demonstrated that simple tooling similar to that used in hot pressing can in fact be used to consolidate articles of complex shape by first surrounding the article by a spherically shaped particulate medium which has sufficient resiliency to be deformed under high compaction pressures without excessive breakage and thereby transmit the compaction pressure in a nearly isostatic manner to the part being consolidated. The Lichti-Hoffstatter patents (U.S. Pat. Nos. 4,539,175 and U.S. Pat. No. 4,640,711) note the advantages provided by using Superior Graphite Co. spherical graphitic carbon product (9400 Series) as the preferred pressure transmitting particulate medium.
Although the consolidation methods of Lichti and Hofstatter theoretically may offer significant advantages in consolidation of preformed articles, it has in actual practice been found to be difficult to apply the described procedure for materials requiring very high temperatures for consolidation; for example, for materials that must be heated to above 1500.degree. C. (2732.degree. F.) to achieve densification. U.S. Pat. No. 4,539,175 to Lichti et. al. notes that the consolidation process takes place after the heated preform is placed in a bed of previously heated carbonaceous particles (Col. 4, lines 35 et seq.). It is further stated that although the graphite particles can be heated inductively to 4000.degree. F. (2204.degree. C.), oxidation is significant above 800.degree. F. Hence, it follows that the operation step at temperatures approaching 4000.degree. F. (2204.degree. C.) would become extremely difficult. U.S. Pat. No. 4,539,175 specifically notes that heating to the necessary temperature is done before compaction.
U.S. Pat. No. 4,640,711 describes again the consolidation process of their invention and notes the use of a non-graphitic spherical carbon and mixtures of graphitic and non-graphitic spherical carbon and mixtures of spherical carbon and/or carbon and ceramic particles as the pressure transmitting medium. U.S. Pat. No. 4,640,711 is again specific in noting that the preform and the bed of particles are to be at elevated temperature before pressurization. Thus, the patented processes of Lichti and Hofstatter require that both the particulate medium and the pre-form be preheated independently and then individually transported to the compaction apparatus in sequence to allow the pre-form to be immersed within the medium prior to compaction. Such transfer of materials after heating becomes extremely difficult at temperatures needed to consolidate non-oxide ceramics, such as silicon carbide (1900.degree. -2150.degree. C.), and apparatus to accomplish the transfer is complex, expensive and wasteful of energy.
It is further recognized that a process has been reported by Eliezer and co-workers at the University of Texas referred to as High-Energy, High-Rate Consolidation which utilizes a very high energy electrical discharge, not through a bed, but rather directly to the pre-form while applying compaction pressure to the preform. In this process, electrical energy is transmitted directly through the material being consolidated, and thus can be used only to consolidate those materials having sufficient electrical conductivity. By using a suitably electrically conductive particulate medium, the present invention is not so restricted and materials with no electrical conductivity can be consolidated by the process hereof. Yet further, since the process described by Eliezer et al. is conducted in a manner similar to hot pressing, it is no more suited than hot pressing for making components of complex shape.
In view of Lichti and Hofstatter and Eliezer et al., it is still another objective of the present invention to utilize the principle of consolidation by compacting a free flowing solid particulate medium as a near isostatic pressure transmitter, but to accomplish heating and compaction essentially simultaneously by a rapid, convenient and energy efficient means.
It is a further object of the present invention to include embodiments utilizing a bed material which is not confined to the narrowly specific bed material of the prior art.